A seed is an embryonic plant enclosed in a protective outer covering. It is the primary means by which flowering plants disseminate their offspring.
Seeds come in many shapes and sizes. Some seeds have fleshy appendages that entice animal dispersers; others have hooks, barbs, or sticky hairs that attach to fur or feathers.
Definition
Seeds are the primary means by which plants (especially angiosperms) disseminate their offspring and are the starting point for new plant growth. They consist of an embryonic plant embryo and food reserve enclosed in a protective outer covering or husk. The nutritive tissue inside the seed is called endosperm. The embryo is either monocotyledon or dicotyledon; in the latter, there may be one or two cotyledon leaves attached to the embryonic axis.
Seeds usually give a seedling a better start than a sporeling because of the larger food reserves in them and their multicellularity. They also have a more durable outer cover and are much easier to transport than spores. Many important foods come from seeds, including wheat, rice and corn (maize); beans, peas, peanuts and other legumes; and vegetables such as tomatoes, cucumbers and broccoli. Seeds are also a major source of cooking oils, lubricants and paints, and stimulants like juniper “berries” used in gin manufacture.
Functions
Seeds provide several functions for the plants that produce them. They protect and nourish the embryo or young plant, disperse a plant to new locations and help it start growing in the right conditions.
The first page on this seed worksheet is a beautiful illustration of a dandelion seed with an area for students to label the parts. The second page is a more independent activity that requires the kids to do some research on their own.
Have them work on this page in small groups. Have them look up what the cotyledons and endosperm are (these are storage tissue, like starch and protein, found in seeds). Then have them compare this to cotton and soybean seeds which are non-endospermic dicots. These use the cotyledons to perform the function of the endosperm in other seeds.
Germination
Germination is the resumption of growth in a seed or spore to transform it into a growing plant. It is the result of a sequence of events that includes water uptake, enzyme activation and metabolic changes.
In the first stage, a germinating seed absorbs water through an area of its shell or coating called the micropyle. This absorption hydrates the embryo and triggers enzyme activity to break down storage compounds that have been locked in the seed for years.
After enzymes break down accumulated DNA damage, the cells of a seed become active and begin dividing. Then a root emerges, followed by a shoot with pale green leaflike structures (cotyledons). This cotyledon enlarges and absorbs light energy to perform photosynthesis, generating the oxygen needed for cellular respiration.
Dormancy
Seed dormancy declares to seeds that it is time for them to stop growing, conserve energy and prepare themselves for freezing temperatures, dry weather, or water and nutrient shortages. This allows them to survive and thrive when conditions are more favorable.
Most seeds require a certain number of hours of “chilling” at temperatures below 0 degrees Celsius to break dormancy. This is a process that ensures the seeds have been sufficiently chilled and that the embryo in the seed has finished developing.
Genetic evidence suggests that a chemical called abscisic acid (ABA) is involved in the induction of dormancy during seed development. Experiments using seed pods from legumes and sections of corn cobs containing kernels have demonstrated that ABA is required at a particular point during embryogenesis to induce dormancy.
Dispersal
As plants are unable to select where their seeds land, they need to rely on seed dispersal for the spread of their offspring. This is typically passive, facilitated by wind, animals and gravity.
Many plants enclose their seeds in fleshy fruits that are attractive to animal dispersers, a process known as endozoochory. Others, such as the nutmeg (Myristica fragrans) tree, use bur and barb structures that catch on fur or feathers. Fossils such as those of a black bear scat or New Zealand moa coprolite demonstrate this type of hitchhiking.
Other seeds can be launched into the air by building up tension inside them, similar to how a catapult stores energy in a taut rope. These are called ballistic seeds. Try dropping a variety of seeds in front of a fan to see which travel the farthest.
